1.1 Field of the Invention
1.2 Description of Related Art
1.2.1 Bacterial Live Vector Vaccines
1.2.2 Attenuated Salmonella typhi as a live vector strain
1.2.3 Plasmid Instability
1.2.4 Plasmid Stabilization Systems
1.2.5 Antibiotic Resistance
1.2.6 Segregational Plasmid Maintenance Functions
1.2.7 Post-Segregational Killing (PSK) Functions
1.2.7.1 Proteic Maintenance System: The phd/doc System
1.2.7.2 Antisense Maintenance System: The hok-sok System
1.2.7.3 Balanced Lethal Systems
2. SUMMARY OF THE INVENTION
3. DEFINITIONS
4. BRIEF DESCRIPTION OF THE DRAWINGS
5. DETAILED DESCRIPTION OF THE INVENTION
5.1 Suicide Vectors
5.2 Plasmid-based Expression of Heterologous Antigens
5.3 Balanced Lethal Systems
5.4 Segregation Limitations
5.5 Catalytic Activity Limitations
5.6 The Non-Catalytic SSB PSK Function
5.7 Expression Plasmids and Self-Contained Genetic Cassettes
5.8 Components of the Antigen Expression and Replication Cassette
5.8.1 Promoter
5.8.2 Origin of Replication
5.8.3 Expressed Protein or Peptide
5.8.4 Heterologous Antigens
5.8.4.1 The Shiga Toxin Family
5.8.5 Site-Specific Mutagensis of Shiga Toxins
5.9 Pharmaceutical Formulations
6. EXAMPLES
6.1 pGen Structure
6.2 POmpC Promoter
6.3 Modified OmpC Promoter
6.4 Origins of Replication and Selection Cassettes
6.5 The Hok-Sok Antisense Post-Segregational Killing Locus
6.6 Complementation-Based Killing System
6.7 Conclusions
7. REFERENCES
THE CLAIMS
ABSTRACT OF THE DISCLOSURE
1.1 Field of the Invention
The present invention relates generally to expression plasmids stabilized by a Plasmid Maintenance System (as defined herein) capable of expressing a protein or peptide, such as an antigen for use in a live vector vaccine, and methods for making and using the stabilized plasmids. The invention optimizes the maintenance of expression plasmids at two independent levels by: (1) removing sole dependence on catalytic balanced lethal maintenance systems; and (2) incorporating a plasmid partition system to prevent random segregation of expression plasmids, thereby enhancing inheritance and stability.
1.2 Description of Related Art
Set forth below is a discussion of art relevant to the present invention.
1.2.1 Bacterial Live Vector Vaccines
Bacterial live vector vaccines represent an important and promising strategy in the field of vaccine development. These vaccines deliver antigens to a host immune system by expressing the antigens from genetic material contained within a bacterial live vector. The genetic material is typically a replicon, such as a plasmid. The antigens may include a wide variety of proteins and/or peptides of bacterial, viral, parasitic or other origin.
Among the bacterial live vectors currently under investigation are attenuated enteric pathogens (e.g., Salmonella typhi, Shigella, Vibrio cholerae), commensals (e.g., Lactobacillus, Streptococcus gordonii) and licensed vaccine strains (e.g., BCG). For the reasons discussed below, S. typhi is a particularly attractive candidate for human vaccination.
1.2.2 Attenuated Salmonella typhi as a Live Vector Strain
S. typhi is a well-tolerated live vector that can deliver multiple unrelated immunogenic antigens to the human immune system. S. typhi live vectors have been shown to elicit antibodies and a cellular immune response to expressed antigen. Examples of antigens successfully delivered by S. typhi include the non-toxigenic yet highly immunogenic fragment C of tetanus toxin and the malaria circumsporozoite protein from Plasmodium falciparum. 
S. typhi is characterized by enteric routes of infection, a quality which can enable oral vaccine delivery. S. typhi also infects monocytes and macrophages and can therefore target antigens to professional APCs.
Expression of an antigen by S. typhi generally requires incorporation of a recombinant plasmid encoding the antigen. Consequently, plasmid stability is a key factor in the development of high quality attenuated vaccines with the ability to consistently express foreign antigens.
Attenuated S. typhi vaccine candidates for use in humans should possess at least two well separated and well defined mutations that independently cause attenuation, since the chance of in vitro reversion of such double mutants would be negligible.
The attenuated vaccine candidate S. typhi CVD908 possesses such properties. CVD908 contains two non-reverting deletion mutations within the aroC and aroD genes. These two genes encode enzymes critical in the biosynthetic pathway leading to synthesis of chorismate, the key precursor required for synthesis of the aromatic amino acids phenylalanine, tyrosine, and tryptophan. Chorismate is also required for the synthesis of p-aminobenzoic acid; after its conversion to tetrahydrofolate, p-aminobenzoic acid is converted to the purine nucleotides ATP and GTP.
1.2.3 Plasmid Instability
Plasmidless bacterial cells tend to accumulate more rapidly than cells containing active plasmids. Summers, The Biology of Plasmids, 65-91, 1996 (incorporated herein by reference). One reason for this increased rate of accumulation is that the transcription and translation of plasmid genes imposes a metabolic burden which slows cell growth and gives plasmidless cells a competitive advantage. Furthermore, foreign plasmid gene products are sometimes toxic to the host cell.
Stable inheritance of plasmids is desirable in the field of attenuated bacterial live vector vaccines to ensure successful continued antigen production, as well as in commercial bioreactor operations in order to prevent bioreactor takeover by plasmidless cells.
Stable inheritance of a plasmid generally requires that: (1) the plasmid must replicate once each generation, (2) copy number deviations must be corrected, and (3) upon cell division, the products of replication must be distributed to both daughter cells. Summers, The Biology of Plasmids, 65-91, 1996 (the entire disclosure of which is incorporated herein by reference).
Although chromosomal integration of foreign genes confers stability to such sequences, the genetic manipulations involved can be difficult, and the drop in copy number of the heterologous gene often results in production of insufficient levels of heterologous antigen to ensure an optimal immune response. Introduction of heterologous genes onto multicopy plasmids maintained within a live vector strain is a natural solution to the copy number problem. Genetic manipulation of such plasmids for controlled expression of such heterologous genes is straightforward; however, resulting plasmids can become unstable in vivo, resulting in loss of these foreign genes.
1.2.4 Plasmid Stabilization Systems
In nature bacterial plasmids are often stably maintained. See Gerdes et al. Annu. Rev. Genet., 31:1-31, 1997 (incorporated herein by reference). In some circumstances, stable maintenance may simply result from a high copy number. However, many proteins, such as antigens, which may be desirably produced by bacterial cells are toxic if produced in large amounts per cell. Therefore, it is desirable to provide stable lower copy number plasmids for use in bacterial cells.
Stable inheritance of naturally occurring lower copy number plasmids can depend on the presence of certain genetic systems which actively prevent the appearance of plasmid-free progeny. A recent review of plasmid stabilization systems can be found in Jensen et al. Molecular Microbiol. 17:205-210, 1995 (incorporated herein by reference).
1.2.5 Antibiotic Resistance
One means for stabilizing plasmids is to provide an antibiotic resistance gene on the plasmid and to grow the cells in antibiotic-enriched media. However, this method is subject to a number of difficulties. The antibiotic resistance approach is expensive, requiring the use of costly antibiotics and perhaps, more importantly, the use of antibiotics in conjunction with in vivo administration of vaccine vectors may promote the growth of antibiotic-resistant bacteria and is currently forbidden by the U.S. Food and Drug Administration.
In large-scale production applications, the use of antibiotics may impose other limitations. With respect to commercial bioreactors, antibiotic resistance mechanisms can degrade the antibiotic and permit a substantial population of plasmidless cells to persist in the culture. Such plasmidless cells are unproductive and decrease the output of the bioreactor.
There is therefore a need in the art for a plasmid stabilization system specifically designed for use in bacterial live vector vaccines which does not rely on antibiotic resistance, and preferably which is also useful in commercial bioreactor applications.
1.2.6 Segregational Plasmid Maintenance Functions
Stable lower copy number plasmids typically employ a partitioning function that actively distributes plasmid copies between daughter cells. Exemplary partitioning functions include, without limitation, systems of pSC101, the F factor, the P1 prophage, and IncFII drug resistance plasmids. Such functions are referred to herein as xe2x80x9cSEGxe2x80x9d functions.
1.2.7 Post-Segregational Killing (PSK) Functions
PSK plasmid maintenance functions typically employ a two component toxin-antitoxin system and generally operate as follows: The plasmid encodes both a toxin and an antitoxin. The antitoxins are less stable than the toxins, which tend to be quite stable. In a plasmidless daughter cell, the toxins and anti-toxins are no longer being produced; however, the less stable antitoxins quickly degrade, thereby freeing the toxin to kill the cell.
The toxins are generally small proteins and and the antitoxins are either small proteins (proteic systems such as phd/doc) or antisense RNAs which bind to the toxin-encoding mRNAs preventing their synthesis (antisense systems such as hok/sok).
Balanced lethal systems discussed below in Section 1.2.6.3 are an example of an artificial PSK function.
1.2.7.1 Proteic Maintenance System: The phd/doc System
In proteic PSK functions, both the toxin and antitoxin are synthesized from operons in which the gene encoding the antitoxin is upstream of the gene encoding the toxin. These operons autoregulate transcription levels, and synthesis of the encoded proteins is translationally coupled. The antitoxin is generally synthesized in excess to ensure that toxin action is blocked. The unstable antitoxins are constantly degraded by host-encoded proteases, requiring constant synthesis of antitoxin to protect the cell. Upon loss of the plasmid, antitoxins are no longer produced and the existing antitoxins rapidly degrade. This frees the toxin to kill the host cell.
The phd/doc system is an example of a proteic PSK function. The phd/doc system occurs naturally within the temperate bacteriophage P1, which lysogenizes Escherichia coli as an xcx9c100 kb plasmid. This maintenance locus encodes two small proteins: the toxic 126 amino acid Doc protein causes death on curing of the plasmid by an unknown mechanism, and the 73 amino acid Phd antitoxin prevents host death, presumably by binding to and blocking the action of Doc.
Phd and Doc are encoded by a single transcript in which the ATG start codon of the downstream doc gene overlaps by one base the TGA stop codon of the upstream phd gene. Expression of these two proteins is therefore translationally coupled, with Phd synthesis exceeding synthesis of the toxic Doc protein.
In addition, transcription of this operon is autoregulated at the level of transcription through the binding of a Phd-Doc protein complex to a site which blocks access of RNA polymerase to the promoter of the operon as concentrations of both proteins reach a critical level. Although Doc appears to be relatively resistant to proteolytic attack, Phd is highly susceptible to cleavage. The PSK mechanism of a plasmid-encoded phd-doc locus is therefore activated when bacteria spontaneously lose this resident plasmid, leading to degradation of the Phd antitoxin and subsequent activation of the Doc toxin which causes cell death.
1.2.7.2 Antisense Maintenance System: The hok-sok System
In antisense maintenance systems, the antitoxins are antisense RNAs that inhibit translation of toxin-encoding mRNAs. Like the antitoxin peptides, the antisense RNAs are less stable than the toxin-encoding mRNA. Loss of the plasmid permits existing antitoxins to degrade, thereby permitting synthesis of the toxin which kills the host cell.
An example of an antisense maintenance system is the hok-sok system, encoded by the parB locus of plasmid R1. See Thisted et al., J. Mol. Biol. 247:859-873, 1995 (incorporated herein by reference). The system is comprised of three genes: hok, sok and mok.
Hok is a membrane-associated protein which irreversibly damages the cell membrane, killing host cells. Expression of Hoc from hok mRNA leads to a loss of cell membrane potential, arrest of respiration, changes in cell morphology, and death.
The sok gene encodes a trans-acting RNA which blocks translation of hok mRNA, thereby preventing Hoc killing of host cells. The sok RNA is less stable than hok mRNA and is expressed from a relatively weak promoter. Gerdes et al. Annu. Rev. Genet., 31:1-31, 1997. The mechanism by which sok RNA blocks translation of Hok in plasmid-containing cells became apparent only after the identification of mok (modulation of killing), a third gene in the parB locus. The mok open reading frame overlaps with hok, and is necessary for expression and regulation of hok translation.
The sok antisense RNA forms a duplex with the 5xe2x80x2 end of the mok-hok message rendering the mok ribosome binding site inaccessible to ribosomes and promoting RNase III cleavage and degradation of the mRNA. In the absence of mok translation, hok is not expressed from intact message, even though its own ribosome binding site is not directly obscured by sok RNA.
When a plasmid-free cell is formed, the unstable sok RNA decays much more rapidly than the stable mok-hok message. When the protection afforded by sok is lost, Mok and Hok are translated and the cell dies.
The difficulty with the hok-sok system is that a significant number of plasmidless cells can arise even when the hok-sok system is operative.
1.2.7.3 Balanced Lethal Systems
In a balanced-lethal system (a PSK function), a chromosomal gene encoding an essential structural protein or enzyme is deleted from the bacterial chromosome or is mutated such that the gene can no longer operate. The removed or damaged gene is then replaced by a plasmid comprising a fully operating gene and a nucleotide sequence encoding the protein or peptide of interest, e.g., an antigen. Loss of the plasmid results in an insufficiency of the essential protein and the death of the plasmidless cell.
A balanced-lethal system has been successfully employed in S. typhimurium based on expression of the asd gene encoding aspartate xcex2-semialdehyde dehydrogenase (Asd). Asd is a critical enzyme involved in the synthesis of L-aspartic-xcex2-semialdehyde, which is a precursor essential for the synthesis of the amino acids L-threonine (and L-isoleucine), L-methionine, and L-lysine, as well as diaminopimelic acid, a key structural component essential to the formation of the cell wall in Gram-negative bacteria. Loss of plasmids encoding such a critical enzyme would be lethal for any bacterium incapable of synthesizing Asd from the chromosome, and would result in lysis of the bacterium due to an inability to correctly assemble the peptidoglycan layer of its cell wall.
The asd system (a PSK function) has been successfully employed in attenuated S. typhimurium-based live vector strains for immunization of mice with a variety of procaryotic and eucaryotic antigens including such diverse antigens as detoxified tetanus toxin fragment C and the LT enterotoxin, synthetic hepatitis B viral peptides, and gamete-specific antigens such as the human sperm antigen SP10.
Murine mucosal immunization with these live vector strains has elicited significant immune responses involving serum IgG and secretory IgA responses at mucosal surfaces.
The asd system has recently been introduced into attenuated Salmonella typhi vaccine strains in an attempt to increase the stability of plasmids expressing synthetic hepatitis B viral peptides. However, when volunteers were immunized with these live vector strains, no immune response to the foreign antigen was detected. In fact, to date, few reports have documented an immune response to plasmid-based expression of a foreign antigen from stabilized plasmids after human vaccination with an attenuated S. typhi live vector.
In one report, the vaccine strain Ty21a was made auxotrophic for thymine by selecting in the presence of trimethoprim for an undefined mutation in the thyA gene, encoding thymidylate synthetase. Although in some cases failure of live vector strains may have resulted from over-attenuation of the strain itself, it appears probable that current killing systems for plasmids suffer from additional limitations.
Those situations where the chromosomal copy of the gene has been inactivated, rather than removed, may allow for restoration of the chromosomal copy via homologous recombination with the plasmid-borne gene copy if the bacterial strain utilized is recombination-proficient.
Balanced-lethal systems based on catalytic enzyme production are subject to a number of important deficiencies. In particular, since complementation of the chromosomal gene deletion requires only a single gene copy, it is inherently difficult to maintain more than a few copies of an expression plasmid. The plasmidless host strain must be grown on special media to chemically complement the existing metabolic deficiency.
Plasmidless cells may also benefit from xe2x80x9ccross-feedingxe2x80x9d effects when a diffusible growth factor is growth limiting.
There is therefore a need in the art for a Plasmid Maintenance System which is not solely reliant on a balanced lethal system, particularly for use in bacterial live vector vaccines.
The present invention relates generally to a stabilized expression plasmid which carries a Plasmid Maintenance System and a nucleotide sequence encoding a protein or peptide, such as a foreign antigen, and methods for making and using the stabilized expression plasmids.
In a particular aspect, the stabilized expression plasmid is employed in a Salmonella typhi live vector vaccine, such as the strain CVD908-htrA.
The invention optimizes the maintenance of expression plasmids at two independent levels by: (1) removing sole dependence on balanced lethal maintenance systems; and (2) incorporating a plasmid partition system to prevent random segregation of expression vector plasmids, thereby enhancing their inheritance and stability. In one aspect of the invention, the stabilized expression plasmid is recombinantly engineered to express one or more antigens, preferably one or more Shiga toxin 2 (Stx2) antigens, such as Shiga toxin subunit pentamers or a genetically detoxified Stx 2.
The stabilized expression plasmid preferably comprises one or more non-catalytic Plasmid Maintenance Functions.
In another aspect, the expression plasmid comprises a Plasmid Maintenance System which comprises at least one PSK function and at least one SEG function. For example, the Plasmid Maintenance System may comprise a two-component Plasmid Maintenance System comprising one PSK function and one SEG function. Alternatively, the Plasmid Maintenance System may comprise a three-component Plasmid Maintenance System comprising a PSK function, a SEG function and another Plasmid Maintenance Function. In a preferred alternative, the Plasmid Maintenance System comprises hok-sok+par+parA.
The Plasmid Maintenance Systems can be incorporated into multicopy expression plasmids encoding one or more proteins or peptides of interest. Such multicopy expression plasmids produce a gene dosage effect which enhances the level of expression of the protein or peptide of interest. Where the Plasmid Maintenance System is to be employed in a bacterial live vector vaccine, the protein or peptide of interest is one or more foreign antigens.
In one aspect, the expression plasmid is a vaccine expression plasmid comprising a Plasmid Maintenance System and at least one antigen, for example, at least one Shiga toxin 2 (Stx2) antigen. Where the antigen is a Shiga toxin 2 antigen, the Shiga toxin 2 antigen can, for example, be either a B subunit pentamer or a genetically detoxified Stx 2.
In another aspect the expression plasmid comprises a Plasmid Maintenance System which incorporates the ssb balanced lethal system and the ssb locus of the bacterial live vector has been inactivated using a suicide vector comprising a temperature sensitive origin of replication. In one aspect, the bacterial live vector is S. typhi and the suicide vectors are used to inactivate the ssb locus of S. typhi. In one aspect, the suicide vector is a derivative of pSC101 which carries sacB, described herein.
In another aspect, the present invention provides a Plasmid Maintenance System incorporating a PSK function involving a silent plasmid addiction system based on antisense RNA control mechanisms that only synthesize lethal proteins after plasmid loss has occurred.
In one aspect the expression plasmid comprises a series of expression plasmids, each comprising self-contained genetic cassettes encoding regulated expression of a heterologous antigen, an origin of replication, and a selectable marker for recovering the plasmid.
In one aspect the expression plasmid comprises a Plasmid Maintenance System which incorporates a PSK function based on the ssb gene. In a related aspect, mutated alleles such as ssb-1, described herein, are incorporated into the expression plasmids to enhance higher copy number plasmids by overexpression of SSB1-like proteins to form the required biologically active tetramers of SSB.
In another aspect, the expression plasmid comprises a promoter. The promoter is preferably an inducible promoter, such as the ompC promoter. In one aspect, the inducible promoter is the mutated PompC1 promoter described herein.
In one aspect, the expression plasmid of the present invention comprises a plasmid inheritance (or partition) locus; an origin of replication selected to provide copy number which effectively stabilizes a given antigen; a PSK function; and a nucleotide sequence encoding an antigen and a promoter which controls translation of the antigen and has a strength which is selected to improve antigen production without killing the cell.
The present invention also provides a method of using the expression plasmid comprising transforming a bacterial cell using said expression plasmid, and culturing the bacterial cell to produce the protein or peptide (e.g., the antigen), and/or administering said transformed cell or cell culture to a subject. Where the transformed bacterial cells are administered to a subject, they are administered in an amount necessary to elicit an immune response which confers immunity to the subject for the protein or peptide. The subject is preferably a human, but may also be another animal, such as a dog, horse, or chicken.
In one aspect, an expression plasmid is provided which comprises at least 3 independently functioning expression cassettes wherein one cassette encodes a protein or peptide of interest and the remaining cassettes each encode a different Plasmid Maintenance Function.
In one aspect, an expression plasmid is provided which encodes (1) a test antigen operably linked to a promoter and (2) a Plasmid Maintenance System.
In another aspect, a regulated test antigen expression cassette is provided which operates such that as induction of antigen expression is increased, a metabolic burden is placed on the bacterium which leads phenotypically to plasmid instability, i.e. a selective advantage is created for all bacteria which can spontaneously lose the offending plasmid. The test antigen can be the green fluorescent protein (GFP). The expression cassette encoding the test antigen can also comprise an inducible promoter, such as the ompC promoter, positioned such that the inducible promoter drives the translation of the test antigen.
In one aspect, a method of making an expression plasmid is provided which comprises synthesizing an expression plasmid comprising at least 3 independently functioning expression cassettes wherein one cassette encodes a protein or peptide of interest and the remaining cassettes each encode a different Plasmid Maintenance Function.
In one aspect, a method of screening Plasmid Maintenance Systems is provided comprising: providing one expression cassette which encodes a protein or peptide of interest, and at least two other expression cassettes, each encoding and capable of expressing in the host bacterial live vector a different Plasmid Maintenance Function; inserting the three expression cassettes into a single expression plasmid; transforming a bacterial live vector with the single expression plasmid; culturing the transformed bacterial live vector; and determining the rate of introduction of plasmidless cells into the culture.
In one aspect, the present invention comprises an attenuated bacterial live vector vaccine comprising an attenuated bacterial live vector which has been transformed with a stabilized expression plasmid comprising a Plasmid Maintenance System, preferably a non-catalytic plasmid maintenance system.
In one aspect, the present invention comprises an attenuated bacterial live vector vaccine comprising an attenuated bacterial live vector which has been transformed with an expression plasmid comprising a Plasmid Maintenance System which incorporates at least one PSK system and one SEG system. The attenuated bacterial live vector can, for example, be S. typhi CVD908-htrA.
The present invention also provides a method for vaccinating a subject comprising administering to the subject an amount of a bacterial live vector vaccine sufficient to elicit an immunity-enhancing immune response. The present invention also provides a method for preventing a disease by vaccinating a subject using an amount of such bacterial live vector sufficient to elicit an immune response to one or more pathogens of such disease. The subject is preferably a human but may also be another animal, such as a horse, cow or pig. For example, the present invention provides a method for preventing hemolytic uremic syndrome (HUS) caused by Shiga toxin 2-producing enterohemorrhagic Escherichia coli by administering to a subject an amount of a bacterial live vector transformed with a stabilized plasmid encoding at least one Shiga toxin 2 antigen.
In another aspect, the present invention provides a method for screening Plasmid Maintenance Systems for efficacy, the method comprising: providing expression plasmids comprising the Plasmid Maintenance Systems described herein and encoding for a protein or peptide of interest, said expression plasmids having copy numbers which vary from low copy number (i.e. xcx9c5 copies per cell) to medium copy number (xcx9c15 copies per cell) to high copy number (xcx9c60 copies per cell); transforming bacterial live vectors with such expression plasmids; and testing for rate of introduction of plasmidless cells and/or rate of growth of plasmid-containing cells. The modified origins of replication may be origins of replication from the plasmids pSC101 (low copy number), pACYC184 (medium copy number), and pAT153 (high copy number). Independently functioning plasmid replication cassettes can be utilized which permit testing of the efficiency of one or more plasmid stabilization systems as copy number is increased.
In another aspect, the present invention provides stabilized expression plasmids for use in attenuated S. typhi live vectors which contain a selectable marker which can readily be replaced by a non-drug resistant locus or by a gene encoding an acceptable drug resistance marker such as aph encoding resistance to the aminoglycosides kanamycin and neomycin.
The constructs of the present invention provide improved stability of recombinant plasmids, overcoming prior art problems of plasmid instability, for example, in bioreactor and live vector vaccination uses. The plasmids of the present invention are specifically tailored for vaccine applications though such plasmids are also useful in large scale protein production.
The plasmids of the present invention are a major improvement over the prior art in that they overcome the problems associated with plasmidless takeover and plasmid instability and have wide ranging utility in fields such as commercial protein production and attenuated bacterial live vector vaccine production.
There has long been a need for a solution to the problems of plasmidless takeover and plasmid stability associated with the field of vaccine delivery and protein production. The present invention is a major step toward the satisfaction of this need.
The term xe2x80x9cPlasmid Maintenance Systemxe2x80x9d (xe2x80x9cPMSxe2x80x9d) as used herein refers to a nucleotide sequence comprising at least one post-segregational killing function (xe2x80x9cPSKxe2x80x9d) and at least one partitioning or segregating system (xe2x80x9cSEGxe2x80x9d).
The term xe2x80x9cPlasmid Maintenance Functionxe2x80x9d is used herein to refer to any function associated with a PMS.
The term xe2x80x9cPost-Segregational Killing Systemxe2x80x9d (PSK) is used herein to refer to any function which results in the death of any newly divided cell which does not inherit the plasmid of interest, and specifically includes balanced-lethal systems such as asd or ssb, proteic systems such as phd/doc and antisense systems such as hok-sok.
The terms xe2x80x9cSegregating Systemxe2x80x9d and/or xe2x80x9cPartitioning Systemxe2x80x9d (both referred to herein as xe2x80x9cSEGxe2x80x9d) are used interchangeably herein to refer to any Plasmid Maintenance function that operates to increase the frequency of successful delivery of a plasmid to each newly divided cell, as compared to the frequency of delivery of a corresponding plasmid without such a SEG system. SEG systems include, for example, equipartitioning systems, pair-site partitioning systems, and the par locus of SC101.
The term xe2x80x9cdetoxifiedxe2x80x9d is used herein to describe a toxin having one or more point mutations which render the toxin non-toxic versus a corresponding toxin without such point mutations.
The term xe2x80x9cimmunizingly effectivexe2x80x9d is used herein to refer to an immune response which confers upon the subject of such immune response a degree of immunological cellular memory with the effect that a secondary response (to the same or a similar toxin) is characterized by one or more of the following characteristics: shorter lag phase in comparison to the lag phase resulting from a corresponding exposure in the absence of immunization, production of antibody which continues for a longer period than production of antibody for a corresponding exposure in the absence of such immunization, a change in the type and quality of antibody produced in comparison to the type and quality of antibody produced from such an exposure in the absence of immunization, a shift in class response, with IgG antibodies appearing in higher concentrations and with greater persistence than IgM, an increased average affinity (binding constant) of the antibodies for the antigen in comparison with the average affinity of antibodies for the antigen from such an exposure in the absence of immunization, and/or other characteristics known in the art to characterize a secondary immune response.